Optically encoded labels using machine readable symbols are well known. For example, U.S. Pat. No. 3,553,438 shows an optically encoded label using a circular wedge data format. U.S. Pat. Nos. 3,971,917 and 3,916,160 show machine readable labels using concentric ring codes to represent stored data. Other types of machine readable labels, such as labels having data encoded in an array of rectangular grids as shown in U.S. Pat. No. 4,286,146, or labels having data encoded in the form of microscopic spot grids as shown in U.S. Pat. No. 4,634,850, are well known in the prior art. In another instance, U.S. Pat. No. 4,488,679 shows an optically encoded label using densely packed multi-colored data.
Other types of machine readable symbols have been developed for many applications. For example, the universal product code (UPC) is a bar code symbology widely used in the U.S. retail industry to identify products at the point of sale, or for inventory control purposes. A bar code is a particular type of machine readable symbol in which data is represented as a series of parallel solid lines, or bars, of varying width and spacing. In the industrial area, other bar codes symbologies have been used for package identification systems. Common bar code symbologies include CODABAR, code 39, interleaved 2 of 5 and code 49.
Existing bar code systems typically lack sufficient data density to accommodate the increasing need to encode more information. It is also desirable to reduce label size, which makes it even more difficult to encode more information. For example, in a package sorting operation, a minimum system requires an optically encoded label containing at least a destination code, such as a zip code of 5 to 9 characters.
However, it is also desirable for the same label to encode additional information such as the name, address and telephone number of the addressee, as well as the class of service, shipper identification, and the like. Increasing the size of the label in order to accommodate more encoded data is not an adequate solution. First, small packages must be sorted as well as large ones, which places rigid constraints on the maximum allowable label size. Second, increasing the label size increases the label cost, which is a significant factor in high volume operations.
The alternative to increasing the label size is to fit more data on the same size label. However, to fit more data on the same size label, the size of the optically encoded features must be reduced. For example, in the case of a bar code label, the bar code size and spacing is reduced in order to encode more data on the same size label.
However, a reduction in bar code size requires significant increases in optical resolution, printing accuracy, illumination power, mechanical complexity, and processing speed. The system resulting from a high density, high speed bar code symbology tends to become both technically and economically impractical.
Higher density machine readable labels containing data arrays instead of bar codes are known. A prior art example of a label containing a higher density data array is shown in U.S. Pat. Nos. 4,874,936 and 4,896,029 where data is encoded in the form of an hexagonal data cell array that contains about 100 characters of information. The prior art hexagonal array system requires a scanner and decoder of sufficient resolution to find, digitize and decode the high density data contained in the label. Due to the small optical features of the label, the prior art hexagonal array system also requires the use of powerful illumination, a high resolution imager, a means for sensing the distance of the label from the imager, and a variable focus lens in order to acquire the image, all of which results in a technically complex and expensive system.